Jacket type BC on aqualung |
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| Acronym | BC or BCD |
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| Other names | Buoyancy control device |
| Uses | Ability to adjust and control the overall buoyancy of the diver |
A buoyancy compensator (or buoyancy control device, BC or BCD), also called a stab jacket, is a piece of diving equipment worn by divers to provide a bladder that can have air added to or vented from so as to establish Neutral buoyancy underwater and buoyancy on the surface, when needed.
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BCs may have the following features:
There are three main types of BC:
An adjustable buoyancy life jacket (ABLJ) is fitted around the neck and onto the chest, secured by straps around the waist and usually between the legs. They are sometimes referred to as "horsecollars" because of their resemblance, and are historically derived from Mae West life jackets issued to World War II flyers. As they were developed in the 1960s they have been largely superseded by wing and vest type BCs, primary because of their tendency to shift the diver's center of gravity with inflation.[4] The ABLJ's location on the diver's chest does means that it does the best job of all BC designs when it comes to floating a distressed, fatigued or unconscious diver face-up on the surface in the event of a problem.
Vest BC, stab jacket, stabiliser jacket, stab, waistcoat or disparagingly "Poodle Vest" BCs are inflatable vests worn by the diver around the upper torso, and also act as the cylinder harness. The air bladder may extend from the back around the diver's sides ("wraparound") or may only cover the back ("back inflation.") Wraparound bladders are favored by some divers because they may make it easier to maintain balance, both submerged and on the surface. However, some designs have had the tendency to squeeze the diver's torso when inflated. Back inflation (wing) BC's do not have this problem but have a greater tendency to float the diver face-down on the surface, which presents a possible hazard in an emergency. Vest BC's, whether back-inflate or wraparound, typically provide up to 25 kilograms of buoyancy (depending on size) and are fairly comfortable to wear. Vest BC's are the most common type among recreational divers because they can integrate buoyancy control, weights, attachment points for auxiliary gear, and cylinder retention in a single piece of gear. The diver need only attach a cylinder and regulator set in order to have a complete scuba set. Some "tech-rec" (technical and recreational) Vest BC's have the ability to carry multiple cylinders.
Wings or BP/W consist of an inflatable bladder worn between the diver's back and the cylinder(s). Wings are not a recent development, but have recently become popular again because of technical diving where they are often used, as the technical diver often carries multiple cylinders on his back and/or strapped to the harness's webbing. The bladder and cylinders are fastened to a backplate which is strapped to the diver. The wing design frees the divers sides and front and allows for a large volume bladder with important lift capacity (60 lbs /30 liter Wings are not uncommon). Elasticated webbing around the bladder is used by some to constrict the bladder when not inflated, although there is serious dispute as to the wisdom of this addition.[5] Heavy equipment such as diving cylinders can be fixed to or slung from the back plate.
The diver needs to be able to establish three states of buoyancy at different stages of a dive:
To achieve negative buoyancy, divers who carry or wear buoyant equipment must be weighted to counteract the bouyancy of both the diver and the equipment.
When underwater, a diver often needs to be neutrally buoyant so that the diver neither sinks nor rises. A state of neutral buoyancy exists when the weight of water that the diver displaces equals the total weight of the diver. The diver uses a BC to maintain this state of neutral buoyancy by adjusting the BC's volume and therefore its buoyancy, in response to various effects, which alter the diver’s overall volume or weight, primarily:
In practice, the diver doesn't think about all this theory during the dive. To remain neutrally buoyant, gas is added to the BC when the diver is negative (too heavy), or vented from the BC when the diver is too buoyant (too light). A feature of diving is that there isn't any automatically stable equilibrium position for a diver wearing a BC, or even simply for a diver with lungs full of air. Any change in depth from a position of neutrality and even a small changes in volume, including the simple act of breathing, result in a force toward an even less neutral depth. Thus, maintenance of neutral buoyancy in scuba must be a continuous and active procedure—the diving equivalent of balance, in a positive feedback environment. Fortunately, the diver's mass provides a source of inertia, as does the liquid medium, so small perturbations (such as from breathing) can be compensated for easily by an experienced diver.
A feature of scuba which is often non-intuitive for beginners, is that gas generally needs to be added to the BC when a diver descends in a controlled manner, and valved-off (removed or vented) from the BC when the diver ascends in a controlled manner. This gas (added or vented) maintains the volume of the gas bubble in the BC during depth changes; this bubble needs to remain at constant volume for the diver to remain even approximately neutrally buoyant. When gas is not added to the BC during a descent, the gas bubble in the BC decreases in volume due to the increasing pressure, resulting in faster and faster descent with depth, until the diver hits the bottom. The same runaway phenomenon, an example of positive feedback, can happen during ascent, resulting in uncontrolled ascent, until a diver prematurely surfaces without a safety (decompression) stop.
With experience, divers learn to minimize this problem, starting by minimizing the size of the "constant volume bubble" in their BC's. This requires learning the minimum weighting requirement needed for their system (see factors above). These techniques keep the volume of the gas bubble within the BC as small as possible at the beginning of a dive, while leaving just enough gas in the BC at first submersion to be able to compensate for the expected slow loss of diver weight as the dive progresses, as a result of gas used (in practice, about 5 to 8 lbs. lost per cylinder, as noted above).
Somewhat complex automatic reflex behaviors are also developed by experienced divers, involving breathing control and BC gas management during depth changes, which allow them to remain neutrally buoyant from minute to minute during a dive, without having to think much about it. Experienced scuba divers may often be identified by the fact that they maintain neutrality without any fin use, as fish do.
See Also: Neutral buoyancy
The vertical-horizontal orientation of the submerged diver is influenced by the BC and by other buoyancy and weight components and contributed to by the diver's body, clothing and equipment. The diver typically wishes to be positioned nearly horizontally (prone) while under water, to be able to see and swim usefully, but more nearly vertical and perhaps partly supine, to be able to breathe without a regulator when on the surface.
The orientation of a static and stable object in water, such as a diver, is determined by its center of buoyancy and its center of mass. At equilibrium, they will be lined up under gravity with the center of buoyancy vertically on a line with, (and preferably above) the center of mass. The diver's overall buoyancy and center of buoyancy can routinely be adjusted by altering the volume of the gas in the BC, lungs and diving suit. The diver's mass on a typical dive does not generally change by what seems like much (see above—a typical dive-resort "aluminum 80" tank at 200 bar contains about 2.8 kg (~6 lbs) of air or nitrox, of which about 2.3 kg (~5 lbs) is typically used in a dive, although any air spaces such as in the BC and in diving suits will expand and shrink with depth pressure. Large changes in buoyancy are of course possible if the weight belt is jettisoned, or a heavy object is picked up.
Generally, the diver has minimal control of the relative position of the center of buoyancy in the BC during a dive, but only its quantity. However the diver can change the buoyancy center by control of his equipment setup, which includes its configuration and weighting locations, which ultimately influence where his effective BC lift is positioned relative to his Center of Gravity.[8]
Traditionally, weight belts or weight systems are worn with the weights on, or close to, the waist and are arranged with a quick release mechanism to allow them to be quickly jettisoned to provide extra buoyancy in an emergency, such belts have the advantage of being able to be shifted fore and aft so as to change the diver's center of mass. BCD systems that integrate the weights into the BCD, can providing added comfort so long as the BCD does not have to be removed from the body of the diver, for example in an underwater emergency such as an entanglement. When a weight integrated BCD is removed on the surface a diver wearing no weight-belt and any type of foamed neoprene wetsuit will remain very buoyant.
By inflating the BC at the surface, the conscious diver may be able to easily float face-up, depending on his equipment configuration choices. A fatigued or unconscious diver can be made to float face up on the surface by adjustment of their buoyancy and weights so the buoyancy raises the top and front of the diver's body and the weights act on the lower and back of the body. An inflated Horsecollar always provides this orientation, but an inflated Vest, and all styles of BP/w generally float the diver face-down because the center of buoyancy is too far from the diver's front. This floating orientation is generally considered undesirable and can be minimized pre-dive by relocation of some of the weights (perhaps of quick release type) further to the rear (such as in pockets close to the diver's cylinder), and avoiding the use of aluminum tanks and using higher density cylinders (typically steel), which similarly moves the center of mass to be further behind the diver, and thus, behind the center of buoyancy. The BC type can also be selected with this factor in mind, selecting a style that moves the center of buoyancy forward, as this accomplishes the same net effect. Any or all of these options can be utilized to trim the system out to its desired characteristics[9] and many factors can contribute, such as the number and position of diving cylinders, the type of diving suit, the position and size of stage cylinders, the size and shape of the diver's body and the wearing of ankle weights, or additional dive equipment. Each of these influence a diver's preferred orientation in the water (horizontal) and on the surface (vertical) to some degree.
In 1957, F. G. Jensen and Willard F. Searle, Jr began testing methods for manual and automatic buoyancy compensation for the United States Navy Experimental Diving Unit (NEDU).[10] In their early tests, they determined that manual systems were more desirable due to the size of the automatic systems.[10] Later that year, the Walter Kiddie and Co. sent a prototype buoyancy compensating tank for use with two cylinders to NEDU for evaluation.[11] The valves of this aluminium tank system leaked and testing was delayed until 1959 when it was recommended for field testing.[11]
The ABLJ was developed by Maurice Fenzy in 1961.[4] Early versions were inflated by mouth underwater. Later versions had their own air inflation cylinder. Some had carbon dioxide inflation cartridges (a holdover, for surface use, of the Mae West flyer's lifejacket). This was abandoned when valves that allowed divers to breathe from the BC's inflation bag were introduced. Since 1969 most modern BCs have used inflation gas from one of the diver's main gas cylinders, in addition to an oral inflation tube which is used at the surface in the event the diver has no high pressure gas left (oral inflation can also be used at depth, in cases where a BC inflator hose develops a leak or problem and requires disconnection). In 1972 Watergill developed the Atpac wing, the first wing style BC and in 1978, ScubaPro released the Stabilizer Jacket, the first Vest BC.
More recent innovations for jacket BCs include weight pouches to adjust attitude underwater, putting the weights on the BC rather than on a weightbelt, integrated regulators, heavily reinforced 1050 denier ballistic nylon. Innovations for backplate and wing include redundant bladders, stainless steel backplates, lightweight soft nylon backplates, and 85 lb lift bladders.
Dive Rite pioneered the first wing for diving twin cylinders in 1985. Competitors in tech diving include Ocean Management Systems. Other SCUBA manufacturers include Sherwood, Zeagle, Scubapro, and Cressisub.
There are other types of equipment worn by divers that affect buoyancy:
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